The present invention relates to novel grease compositions which contain fluorinated oils, boron nitride powders and, optionally, solid fluorinated polymers. Such compositions have been found to have exceptionally good lubricating properties, even under severe conditions.
Boron nitride powders are known to be employed because they lower the coefficient of friction of metals which move in contact with one another. The use of hexagonal lattice boron nitride (HLBN) powder in grease compositions based on silicon oils is also known in the art. For instance, Japanese patent J62043493 discloses a grease composition comprising a polyorganosiloxane and boron nitride powder. This reference, however, does not disclose the use of a fluorinated polymer oils, the combination of a solid fluorinated polymer with boron nitride for fluorinated greases, nor a bimodal distribution of boron nitride.
Similarly, United States Air Force report ASDTDR 63-656, part 1, titled "Rheology of Silicone Fluids Thickened with Boron Nitride" (1963) discloses grease compositions comprising a hydrogenated silicone fluid with a density of 1.02 g/ml combined with boron nitride powders. It is disclosed therein that 43.2 to 43.9 weight percent of a standard coarse grain boron nitride is required to form a consistent grease while only 14.5 to 19.7 weight percent of a submicron particle size boron nitride is necessary to form desirable greases. This reference, however, does not describe the use of a fluorinated silicone fluid since the density of fluorinated silicone fluid exceeds 1.02 g/mL (it is about 1.3 g/mL), the use of boron nitride powder having a bimodal particle size distribution, nor the combination of a fluorinated polymer with boron nitride for obtaining fluorinated greases based on fluorinated polymer oils.
The problem to be solved by the present invention is to find grease compositions based on boron nitride having exceptionally good lubricating properties even under severe operating conditions. The solution has been found by the grease compositions as described below.